Removing heat from a relatively constant heat generating system is often required, either to prevent overheating or to provide cooling. One cooling method presently in use is the circulation of a heat transport fluid, such as water, through the system to extract heat. The heat transport fluid is circulated to a cooling location where the heat is extracted. The heat transport fluid circulates in a closed loop system, extracting heat at one location and giving off heat at another location in the system as it circulates. A water-cooled automobile engine using water as the heat transport fluid, a radiator as a heat exchanger, and air as the cooling fluid at the cooling location is an example of this type of heat removal system.
A second type of system is one in which the heat required to be removed from the system fluctuates greatly. For example, a transatmospheric vehicle reentering the atmosphere has a short-term need for a great amount of cooling, even though the cooling requirements while in space may be minimal. A nuclear reactor or large refrigeration unit, such as on a refrigeration truck traveling in alternating hot and cold weather, are other examples of systems whose cooling requirements may fluctuate greatly.
Use of a cryogen cooling fluid to extract heat from the heat transport fluid has been proposed. This is attractive because the low temperature of a cryogen fluid provides greater cooling of the heat transport fluid than is possible using air or water as the cooling fluid. The extraction of large amounts of heat from the heat transport fluid at the cooling location permits smaller heat exchangers to be used.
One disadvantage of using cryogen for the cooling fluid is so much heat may be removed from the heat transport fluid that the heat transport fluid may be frozen. If the heat transport fluid is frozen, circulation is blocked and the system will overheat.
One proposal to prevent freezing of the heat transport fluid is to use temperature feedback from the heat transport fluid to control the flow of the cryogen cooling fluid past the cooling location. According to this proposal, if the temperature of the heat transport fluid is too low, a valve in the flow system of the cryogenic cooling fluid is closed to either restrict the flow volume or slow the rate of flow of the cryogen fluid at the cooling location. Conversely, if the temperature of the heat transport fluid is too high, the valve is opened further to increase the flow volume. This particular solution has the disadvantage of causing cryogen fluid pressure differences or flow oscillations in the cryogen fluid system. Variations in the fluid pressure of a cryogen, such as hydrogen, cause significant changes in the cryogen temperature, heat transfer characteristics, and other thermal properties. This results in the heat extracted from the heat transport fluid being very difficult to predict and control. The heat transport fluid may be frozen at one moment and very hot the next moment due to fluctuations in the thermal properties of the cryogen cooling fluid, even though the system heat generation parameters have not changed. System instability may result if proper controls are not provided. Even if proper controls are provided, they increase the system complexity, weight, and cost.